LANSDOWNE – A section of Highway 401 is closed after a massive pileup involving passenger cars and transport trucks, west of Brockville.

Exactly how many vehicles involved is not known but the highway is shut down in both directions as of roughly 2 p.m.

The situation is being described as a “mass casualty event” by Gananoque police, though the law enforcement agency is not directly involved.

Leeds County O.P.P. say one of the transport trucks involved is leaking a toxic substance and the area is being evacuated as a precaution.

The material turns into “highly toxic” hydrofluoric acid when exposed to heat.

Several patients have been transported to the Lansdowne fire department hall where a decontamination center has been set up. Some of the other injured are being transported to Kingston.

Frontenac County paramedics, supporting Leeds-Grenville paramedics, tweeted that they were transporting 18 patients to hospital for assessment.

Kingston General Hospital was in a Code Orange, indicating an external disaster with mass casualties.

There are reports that several people, including police officers, have been exposed to the liquid and are being treated by medics.

The section of highway closed is between Mallorytown Road and Reynolds Road.

The Township of Leeds and the Thousand Islands says its Community Control Group has been assembled to respond to the pileup.

“Emergency responders are on-site and coordinating medical response. We reiterate the need for people to remain in their cars until otherwise advised. Residents and travellers are asked to stay home if possible or use alternate travel routes,” the township said in a statement.